651
Hydrogen Production
23.7.3 memBrAne systems
Membrane systems separate gases by taking advantage of the difference in rates of diffusion
through membranes (Brüschke, 1995, 2003). Gases that diffuse faster (including hydrogen) become
the permeate stream and are available at low pressure, whereas the slower-diffusing gases become
the non-permeate and leave the unit at a pressure close to the pressure of the feedstock at entry
point. Membrane systems contain no moving parts or switch valves and have potentially very high
reliability. The major threat is from components in the gas (such as aromatics) that attack the membranes, or from liquids, which plug them.
Membranes arc fabricated in relatively small modules; for larger capacity, more modules are
added. Cost is therefore virtually linear with capacity, making them more competitive at lower
capacities. The design of membrane systems involves a trade-off between pressure drop (or diffusion rate) and surface area, as well as between product purity and recovery. As the surface area is
increased, the recovery of fast components increases; however, more of the slow components are
recovered, which lowers the purity.
23.7.4 CryogenIC sePArAtIon
Cryogenic separation units operate by cooling the gas and condensing some, or all, of the constituents for the gas stream. Depending on the product purity required, separation may involve flashing
or distillation. Cryogenic units offer the advantage of being able to separate a variety of products
from a single feed stream. One specific example is the separation of light olefins from a hydrogen
stream.
Hydrogen recovery is in the range of 95%, with purity above 98% obtainable.
23.8 HYDROGEN MANAGEMENT
Many existing refinery hydrogen plants use a conventional process, which produces a mediumpurity (94%–97%) hydrogen product by removing the carbon dioxide in an absorption system and
methanation of any remaining carbon oxides. Since the 1980s, most hydrogen plants are built with
PSA technology to recover and purify the hydrogen to purities above 99.9%. Since many refinery
hydrogen plants utilize refinery off-gas feeds containing hydrogen, the actual maximum hydrogen
capacity that can be synthesized via steam reforming is not certain since the hydrogen content of the
off-gas can change due to operational changes in the hydrotreaters.
Hydrogen management has become a priority in current refinery operations and when planning to produce lower sulfur gasoline and diesel fuels (Zagoria et al., 2003; Davis and Patel, 2004;
Luckwal and Mandal, 2009). Along with increased hydrogen consumption for deeper hydrotreating,
additional hydrogen is needed for processing heavier and higher sulfur crude slates. In many refineries, hydroprocessing capacity and the associated hydrogen network is limiting refinery throughput and operating margins. Furthermore, higher hydrogen purities within the refinery network are
becoming more important to boost hydrotreater capacity, achieve product value improvements, and
lengthen catalyst life cycles.
Improved hydrogen utilization and expanded or new sources for refinery hydrogen and hydrogen
purity optimization are now required to meet the needs of the future transportation fuel market and
the drive toward higher refinery profitability. Many refineries developing hydrogen management
programs fit into the two general categories of either a catalytic reformer supplied network or an
on-purpose hydrogen supply.
Some refineries depend solely on catalytic reformer(s) as their source of hydrogen for hydrotreating. Often, they are semi-regenerative reformers where off-gas hydrogen quantity, purity, and availability change with feed naphtha quality, as octane requirements change seasonally, and when
the reformer catalyst progresses from start-of-run to end-of-run conditions and then goes off-line
Hydrogen Production
23.7.3 memBrAne systems
Membrane systems separate gases by taking advantage of the difference in rates of diffusion
through membranes (Brüschke, 1995, 2003). Gases that diffuse faster (including hydrogen) become
the permeate stream and are available at low pressure, whereas the slower-diffusing gases become
the non-permeate and leave the unit at a pressure close to the pressure of the feedstock at entry
point. Membrane systems contain no moving parts or switch valves and have potentially very high
reliability. The major threat is from components in the gas (such as aromatics) that attack the membranes, or from liquids, which plug them.
Membranes arc fabricated in relatively small modules; for larger capacity, more modules are
added. Cost is therefore virtually linear with capacity, making them more competitive at lower
capacities. The design of membrane systems involves a trade-off between pressure drop (or diffusion rate) and surface area, as well as between product purity and recovery. As the surface area is
increased, the recovery of fast components increases; however, more of the slow components are
recovered, which lowers the purity.
23.7.4 CryogenIC sePArAtIon
Cryogenic separation units operate by cooling the gas and condensing some, or all, of the constituents for the gas stream. Depending on the product purity required, separation may involve flashing
or distillation. Cryogenic units offer the advantage of being able to separate a variety of products
from a single feed stream. One specific example is the separation of light olefins from a hydrogen
stream.
Hydrogen recovery is in the range of 95%, with purity above 98% obtainable.
23.8 HYDROGEN MANAGEMENT
Many existing refinery hydrogen plants use a conventional process, which produces a mediumpurity (94%–97%) hydrogen product by removing the carbon dioxide in an absorption system and
methanation of any remaining carbon oxides. Since the 1980s, most hydrogen plants are built with
PSA technology to recover and purify the hydrogen to purities above 99.9%. Since many refinery
hydrogen plants utilize refinery off-gas feeds containing hydrogen, the actual maximum hydrogen
capacity that can be synthesized via steam reforming is not certain since the hydrogen content of the
off-gas can change due to operational changes in the hydrotreaters.
Hydrogen management has become a priority in current refinery operations and when planning to produce lower sulfur gasoline and diesel fuels (Zagoria et al., 2003; Davis and Patel, 2004;
Luckwal and Mandal, 2009). Along with increased hydrogen consumption for deeper hydrotreating,
additional hydrogen is needed for processing heavier and higher sulfur crude slates. In many refineries, hydroprocessing capacity and the associated hydrogen network is limiting refinery throughput and operating margins. Furthermore, higher hydrogen purities within the refinery network are
becoming more important to boost hydrotreater capacity, achieve product value improvements, and
lengthen catalyst life cycles.
Improved hydrogen utilization and expanded or new sources for refinery hydrogen and hydrogen
purity optimization are now required to meet the needs of the future transportation fuel market and
the drive toward higher refinery profitability. Many refineries developing hydrogen management
programs fit into the two general categories of either a catalytic reformer supplied network or an
on-purpose hydrogen supply.
Some refineries depend solely on catalytic reformer(s) as their source of hydrogen for hydrotreating. Often, they are semi-regenerative reformers where off-gas hydrogen quantity, purity, and availability change with feed naphtha quality, as octane requirements change seasonally, and when
the reformer catalyst progresses from start-of-run to end-of-run conditions and then goes off-line